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Sump buried, main tank has 80mm of concrete with a brick facing.
Very stable temps takes several days for it to change.
Follows the mean temp fairly well. ie Several days 34+ to 37deg and tank recorded a max of 27deg.

A first experiment with insulation of the type WAL100 suggests confirms his experience. I used only two small metal tanks (not the ones on my actual grows bed because I'm in the process of overhauling the whole system, just buried a 65 gallon heat sink of PVC pipes), one insulated the other not, then applied ambient heat to both. The pink batts insulation reduced temperature by twelve percent.

One word to the wise, that I figured out in my research be careful with insulation batts.

Fiberglass batts if they get wet will not insulate as well and may grow mold and other things inside them too. Rockwool batts do not have these problems and would be a better choice if you have them available in your area.

On covering the tank you have to strike a balance. The surface of the tank right now is acting as a degassing mechanism in your system covering it will reduce or almost eliminate this. If you have other areas or mechanism that are degassing and your water is cycling constantly through those areas it might be possible. You just do not want to trap nitrogen and other gasses that become harmful to the fish at elevated levels in the water.

Little known fact fish can also get the bends (divers sickness) from too much nitrogen in the body, but that is most likely not a problem for you. Aquaculturist figured this out trying to get more oxygen into the water with pure air. The longer the air is under water after you release it the more gases that are dissolved.

They were able to get more oxygen into the water by releasing the a 10 meters down from the surface but doing this for a prolonged period of time also increased the nitrogen levels to harmful levels for the fish.